1. Field of the Invention
This invention relates to an additional signal multiplexing device for multiplexing an additional signal in a vertical overscanning area of a main signal, for example, and an additional signal separation device for separating the additional signal from the main signal on which the additional signal is superposed.
2. Description of the Related Art
As is well known in the art, in commercially available television receivers, an overscanning area is generally provided. That is, in a case where the whole size of the display image plane created by television signals is indicated by broken lines in FIG. 1, the actual display area of the television receiver is set to a slightly smaller size as shown by a solid line in FIG. 1. Therefore, an image on the peripheral portion of the entire display image plane area is omitted by the overscanning operation.
The reason why the overscanning area is provided is to prevent that the image plane is partly cut out by variation in the television signal receiving condition or the performance of the television receiver, thus making the viewers unpleasant.
However, in recent years, the performance of television receivers is enhanced and the operation thereof becomes stable, and the overscanning rate of 8% which is normally set can be satisfactorily attained with some margin. As a result, it has been proposed to transmit an additional signal by use of the overscanning area (refer to "STUDY ON TRANSMISSION SYSTEM FOR WIDE ASPECT IMAGE" in the technical report of television institute on Nov. 29, 1988, for example).
That is, a television signal includes 482 effective scanning lines for each frame in the vertical direction (in practice, 483 scanning lines are provided, but since one of them is used for multiplexed text broadcasting, 482 scanning lines can be effectively used), and 482.times.(8/100)=38 of the scanning lines are used for multiplexing new additional signals. Therefore, not the television signal but the additional signal is superposed on 19 (=38/2) of the 38 scanning lines which lie on each of the upper and lower portions of the image plane.
The above additional signal may include side panel components for increasing the width of the image plane (refer to "A Wide Screen EDTV" IEEE Transaction on Consumer Electronics, Vol. 35, No. 3, P.133-P.141, AUGUST 1989), high-definition components of the luminance signal Y and chrominance signal C (refer to "Extended Definition TV Fully Compatible with Existing Standard" IEEE Transactions on Communications, Vol. COM-32, No. 8, P.948-P.953, AUGUST 1984), and helper signals for sequential scanning/conversion (refer to "Encoding for Compatibility and Recoverability in the ACTV System" IEEE Transactions on Broadcasting, Vol. BC-33, No. 4, P.116-P.123, DECEMBER 1987).
However, when the additional signals are superposed on all of the 19 scanning lines on each of the upper and lower portions of the image plane, no margin can be taken for the overscanning in the vertical direction. As a result, when the central value in the vertical deflection is deviated, the image plane may be partly cut out as shown by a hatched portion in FIG. 2. In this case, the partial cut-out of the image plane also occurs when the vertical deflection amplitude is lowered and the raster is slightly narrowed in the vertical direction.
In view of the above fact, the number of scanning lines on which the additional signals are multiplexed on each of the upper and lower portions is limited to about 7 in the prior art so as to prevent the image plane from being partly cut out even when the vertical deflection center is deviated or the vertical deflection amplitude is lowered.